being required, even though the electrolyte is heated to 70 C. and the bipolar electrodes are only % in. apart.
When potassium chloride is employed as electrolyte, the chlorate can be easily separated by crystallization on cooling from the mother-liquor containing the unaltered chloride: but when sodium chlorate is being produced, different treatment is required to obtain separation of the chloride and chlorate, since the sodium salt is much more soluble than potassium chlorate. The current efficiency when producing either sodium or potassium chlorate can be raised to 90%. when the process is well-managed; and the conversion of chloride into chlorate is completed in one operation.
Were it not for the fact that no satisfactory substitute for plati- num can be found as electrode material for chlorate cells, and that consequently the capital costs of the cell installation are very high, the electrolytic process of chlorate manufacture would before now have quite supplanted the older chemical process, which is still operated in the few places where the Le Blanc process survives.
The conditions required in order to obta_in perchlorates, and other highly oxidized salts such as persulphates, in an electrolytic cell are: (l) Insoluble electrodes; (2) a high current density at the anode; (3) the prevention of any reducing action by the hydrogen liberated at the cathode. This latter condition is obtained, either by the use of a diaphragm between the two compartments of the cell, or by the employment of salts, such as chromates or cyanides, which serve to suppress the cathodic reduction.
According to the most reliable information, perchlorates are now produced by electrolyzing a concentrated solution of sodium chlorate, containing 600-700 grammes per litre of this salt, at a temperature of 10 to 30 C. with a current density of 400-500 amperes per sq. foot. When the chlorate content of the electrolyte falls below 100 grammes per litre, the resistance of the bath increases considerably, and the temperature rises to 45 to 50 C. with a reduction of the current density to 270 amperes per sq. foot. If the chlorate concentration of the electrolyte falls below ip grammes per litre, much ozone is given off, and the evolution of this gas may increase to such an extent that the workmen in the cell rooms are affected injuriously. Under normal conditions, however, the conversion of the chlorate into perchlorate proceeds without evolution of ozone, and with an average efficiency of 85%; the power required to produce I kgm. of sodium perchlorate from chlorate being 35 K.W.-hrs. As the sodium salt is deliquescent it is not worked up as such, but the potassium salt is precipitated by adding potassium chloride. The ammonium salt is prepared similarly, by treating the sodium perchlorate solution with ammonium chloride; or the ammonium salt can be produced by starting with calcium chloride, and by converting this by successive stages of anodic oxidation into calcium perchlorate, which is finally decomposed by the ammonium chloride, to yield ammonium perchlorate.
Sodium perborate is produced in a similar manner, by electrolyzing solutions of sodium borate ; and as this salt has found a wide applica- tion in the arts and industries, some further details of its method of manufacture may be given here. The process employed is based on the use of a mixed solution of sodium borate and an alkaline car- bonate as electrolyte, with the addition of some substance which will coat the cathode with a colloidal or other deposit that lowers 'the reducing action of any hydrogen liberated at this point; chromic acid being most suitable for this purpose. During the electrolysis, the strength of the alkali carbonate solution must be maintained, preferably by the presence of the solid salt ; but towards the end of the electrolysis this solid carbonate can be allowed to go into solu- tion. Sodium borate must also be present in the solid state during the electrolysis, in order to keep up the concentration of the elec- trolyte. The presence of magnesium silicate, stannic acid and alkali bicarbonates is said to accelerate the conversion of the borate into the persalt. The latest theory of the conversion is, that percarbonate is first formed, and that this salt then reacts with the sodium borate to yield sodium perborate during the course of the electrolysis; in any case, for the success of this method of production, the presence of solid perborate in the cell appears to be necessary.
Persulphates and Hydrogen Peroxide. Persulphates are an- other class of highly oxidized salts which are finding a wide application in the arts and industries, especially in photography, and here again the electrolytic method of production is the simplest and most efficient.
As a practical matter, in 'judging the comparative merits of the chemical and electrolytic methods for producing these per- salts and compounds, it is necessary to note that the cost of the electric current, when producing pure chemicals of this type, is not a very serious item in the total cost of production. As a rule, the desired salt can be produced in a pure state by one simple operation in the electrolytic cell or bath; and this of course is a factor in the economy of the electrolytic methods which must not be overlooked in judging the comparative cost of electrolytic and chemical methods of manufacture of what are usually classed as " fine " chemicals.
The method of producing persulphates in the electrolytic cell is based upon the discharge of SO 4 ions at the anode of the cell. The conditions required to effect this discharge are the same as in the case of the production of perchlorates: (a) A low temperature; (6) a high current density with a smooth platinum anode; (c) an acid solution, or one at least free from alkali.
When employing a diaphragm type of cell, a concentrated solution of ammonium sulphate is employed in the anode compartment of the cell and sulphuric acid of medium strength in the cathode compart- ment. Smooth platinum must be employed as anode material, but lead can be employed as cathode ; and the cathodes may be of much larger surface area than the anode.
Two methods have been employed for the production of hydrogen peroxide by electrolysis ; the one based upon the use of persulphuric acid as intermediate product, and the other upon the use of potas- sium or ammonium persulphate. The conditions which necessarily should be observed in the first method are as follows: (i) The density of the sulphuric acid should be between I -35 and 1-50; (2) the electrolysis should be carried out as rapidly as possible; (3) the cur- rent density should be high, about 950 amperes per sq. ft. ; (4) the solution must be cooled, or a hollow anode may be used, cooled internally by the circulation of water at 15 to 20 C.
Hypochlorites. It is noteworthy that Charles Watt, in his very remarkable patent No. 13,755 of 1851, clearly explained all the conditions which must be maintained in the cell in order to produce hypochlorites by the electrolytic decomposition of sodium or potassium chloride solutions. The advances that have been made since that year have been simply in the form and design of the apparatus used for carrying out the electrolytic method. The leading features of the modern cells, designed specially for hypochlorite production, are, however, very similar. They all possess graphite or platinum electrodes, placed close together so that the chlorine liberated at the anode reacts at once with the alkaline hydrate formed at the cathode; and they possess also some mechanism for promoting the rapid circulation and cooling of the electrolyte, in order to avoid the formation of chlorate. The three leading types of electrolyzer, for the production of bleaching solutions, are: (i) the Haas and Oettel (or " Manchester," as it is now called), (2) the Kellner, and (3) the Mather and Platt.
The latest form of the Manchester electrolyzer, as operated in 192 1 , makes use of carbon as electrode material, and of the liberation of hydrogen at the cathode, to effect automatic circulation and mixing of the liquid. The inner or working cell is a rectangular stoneware tank, divided by the carbons into 30 narrow compartments or cells. The first and last carbons of the set form the main electrodes of the cell; the intervening carbons act as secondary electrodes, i.e. as anodes on the one face and cathodes on the other. This electrolyzer holds 750 litres of 15% brine, and takes 75 to 80 amperes at no volts. Its output in 10 hours equals 10-5 kgm. of active chlorine.
The Kellner electrolyzer consists of a shallow stoneware tank, divided into a large number of narrow cells by means of vertical glass plates, so arranged that the electrolyte is obliged to take a zig-zag course in its passage through the electrolyzer. The electrodes are formed of platinum-iridium gauze and are arranged horizontally, with the anodes below the cathodes, so that the chlorine liberated at the former may be absorbed by the supernatant liquid. This elec- trolyzer is constructed usually to take a current at 1 10 volts, and has only two terminal electrodes; all the intervening electrodes function as secondary electrodes. The Kellner electrolyzer holds 820 litres of brine testing 15%, and requires a go-ampere current at Iio volts. Its output in 10 hours equals 15-0 kgm.
The Mather and Platt electrolyzer is constructed on the filterpress principle, with a trough for supply of the brine running along the top of the frame. The frame holds 22 separate cells fixed transversely, and the brine feeds these through a perforated tray in fine streams which break up into drops on falling and thus prevent current leak- age. The 22 cells forming one electrolyzer are placed in one frame, and are connected in series. They take a current of 250 amperes at no volts. The salt consumption is 10-3 Ib. salt, and the power consumption is 2i K.W.-hrs. per Ib. of available chlorine produced.
In recent years the direct production of hypochlorites and bleaching solutions by electrolysis has been curtailed. It has been found more economical to use the chlorine gas obtained from the electrolytic alkali cells for this purpose, and to absorb this either in the milk of lime, or in the hydrate solution produced at the cathode, the absorption taking place in a separate vessel outside the cell.
Oxygen and Hydrogen. The electrolysis of acidulated water to show that it is composed of two gases, and the recombination of these gases by explosion to form drops of water, is one of the oldest of chemical lecture-table experiments; and it is not sur- prising, therefore, that cells for producing oxygen and hydrogen upon a commercial scale, by electrolytic methods, have been
